Effect of vegetation cover and sediment type on 3D subsurface structure and shear strength in saltmarshes

Abstract

The vulnerability of saltmarshes to lateral erosion at their margin depends on the local biogeomorphological properties of the substrate. In particular, the 3D architecture of pore and root systems is expected to influence shear strength, with repercussions for the wider-scale stability of saltmarshes. We apply X-ray computed microtomography (μCT) to visualize and quantify subsurface structures in two UK saltmarshes at Tillingham Farm, Essex (silt/clay rich substrate) and Warton Sands (sand-rich substrate), with four types of ground cover: bare ground, Spartina spp, Salicornia spp and Puccinellia spp. We extracted μCT structural parameters that characterize pore and root morphologies at each station, and compared them with field measurements of shear strength using a principal component analysis and correlation tests. The 3D volumes show that species-dependent variations in root structures, plant colonization events and bioturbation activity control the morphology of macropores, while sediment cohesivity determines the structural stability and persistence of these pore structures over time, even after the vegetation has died. Areas of high porosity and high mean pore thickness were correlated to lower values of shear strength, especially at Tillingham Farm, where wellconnected vertical systems of macropores were associated with current or previous colonization by Spartina spp. However, while well-connected systems of macropores may lower the local deformation threshold of the sediment, they also encourage drainage, promote vegetation growth and reduce the marsh vulnerability to hydrodynamic forces. The highest values of shear strength at both sites were found under Puccinellia spp, and were associated with a high density of mesh-like root structures that bind the sediment and resist deformation. Future studies of marsh stability should ideally consider time series of vegetation cover, especially in silt/clay-dominated saltmarshes, in order to consider the potential effect of preserved buried networks of macropores on water circulation, marsh functioning and cliff-face erosion.

In-situ loading experiments reveal how the subsurface affects coastal marsh survival

“…4d) suggesting s ∼10. We believe that this effect is mainly associated to the presence of the in situ root structure of the living vegetation cover present in the upper 0.20-0.25 m. This is in agreement with recent reports on the beneficial effects exerted by halophytes roots and rhizomes on marsh shear and compressibility strength 18,57 .…”

Modeling Decadal Salt Marsh Development: Variability of the Salt Marsh Edge Under Influence of Waves and Sediment Availability

“…The latter species grows faster initially, but its biomass declines over the winter period, and hence explicit seasonality is required to model this species. For inclusion of different species and to improve single‐species simulations, belowground feedback between morphology and vegetation, that is, spatio‐temporal differences in substrate strength (Chirol et al., 2021), is required. This can be simulated by implementing a spatio‐temporal varying critical bed shear stresses.…”

How strong are salt marshes? Geotechnical properties of coastal wetland soils

“…Cohesion in sandy loam agricultural soils also tends to be higher, with Mouazen (2002) considerable increases in cohesion due to the presence of roots. However, the salt marsh samples used here contained not only roots but likely also macropores, which are known to be related to a lower shear strength at these sites (Chirol et al, 2021) and the interpretation of results is necessarily more complicated.…”